1. Field of the Invention
The present invention relates to an improvement in a sundial and more particularly, but not by way of limitation, to a sundial which is correctable with regard to its mounting location and is further correctable for adjusting real solar time to mean solar time and to compensate for legal time such as daylight savings time.
2. Description of the Prior Art
Various types of sundials and celestial observation instruments have been in use for centuries, and with each refinement of these instruments, man has learned more about the movement of the earth with respect to other celestial bodies.
The development of the calendar and, subsequently the present day clocks, are the result of many years of observation, study, and trial and error tests in an attempt to define an an arbitrary time interval which is useful in determining the happening events, and at the same time coincide with the periodic movement of the earth with respect to the stars and particularly earth's sun.
However, our timekeeping system accurate as it may be does not accurately coincide with the movement of the earth with respect to the sun. We have defined a day as being the time required for the earth to rotate about its axis one time with respect to the sun.
Even though the earth's rotation with respect to far away stars is fairly constant, its rotation with respect to the sun is not. A rather large deviation is due to the fact that the earth revolves around the sun, not in a circular orbit, but in an elliptical orbit with the sun substantially located at one of the foci of the ellipse. The elliptical orbit follows Kepler's laws of planetary motion and the earth moves at a greater velocity when near the perihelion of its orbit than when near the aphelion. Therefore, since the earth's rotation about its own axis is at a rather constant velocity, the real solar day or the interval between two successive identical positions of the sun with respect to the earth varies as the earth moves along its elliptical path. Hence, a mean solar day was defined as being equal to 24 hours where the hour is based on the length of a mean solar day, averaging said lengths over a year's period of time.
Therefore, the mean solar day coincides with a real solar day only at certain positions on its elliptical path. The positions where real solar time coincides with mean solar time occurs near the dates of Dec. 24, Apr. 14, June 15, and Sept. 2. Between Dec. 24 and Apr. 14, mean solar time leads real solar time by as much as 13 minutes. Between Apr. 14 and June 15, mean solar time trails or lags behind real solar time by as much as 131/2 minutes. Between June 15 and Sept. 2, mean solar time again leads real solar time by as much as 61/2 minutes, and between Sept. 1 and Dec. 24, mean solar time again lags behind real solar time by as much as 151/2 minutes.
Further, since there is a mean solar time change of 1 hour corresponding to every 15.degree. of displacement of longitude from the Greenwich meridian, it would be impractical for one to compensate for his exact longitudinal position. Thus, one hour time zones have been established which are approximately 15.degree. in longitude apart around the entire globe. Therefore, if one is situated at Tulsa, Oklahoma which is approximately 96.degree. longitude west or 6.degree. west of the U.S. standard 90.degree. west meridian, his clock would be 6/15 hour or 24 minutes ahead of mean solar time. Hence, a typical sundial located in Tulsa, Oklahoma, to measure solar time, would indicate approximately 11:36 a.m. while an observer's clock would indicate 12 o'clock noon
Presently available sundials, while possible adjustable with respect to latitude location, are not capable of being adjusted to either mean solar time or locations within a time zone. Neither are such known sundials capable of adjustment for legal time changes such as daylight savings time.